he street, while I hold the ball, and
let the string unwind."

Rollo did so. He drew out a long piece of twine, as long as the whole
front of the house, and then he stopped to ask his father if that was
enough.

"No," said his father; "walk along."

So Rollo walked on for some distance farther, until, at last, the ball
was entirely unwound. Rollo had one end of it, and was standing at some
distance down the road, while his father, with the other end, stood at
the gate of the front yard. The middle of the string hung down pretty
near to the ground.

"Draw tight, Rollo," said his father.

So Rollo pulled a little harder, and by that means drew the line
straighter.

"Now," said his father, "walk along slowly."

So Rollo walked along, drawing the end of the line with him. His father
followed with the other end. Thus they advanced several steps along the
side of the road.

"There," said his father. "Stop. That, you see, was a _progressive_
motion."

"Yes, sir," replied Rollo.

"The whole string advanced along the road," added his father. "It made
progress, and so it was a progressive motion. Now, fasten your end of
the string, Rollo, to that tree directly behind you."

Rollo looked behind him, and saw that he was standing near a small
maple-tree, which had been planted, a few years before, by the side of
the road.

"Tie it right around the stem of the tree," said his father, "about as
high as your shoulder."

Rollo fastened the string as his father had directed. Then his father
fastened _his_ end, in the same way, to another tree, which was growing
near where he was standing.

"Now," said he, "there can be no more progressive motion, but there can
be a vibratory one. Take hold of the string near where it is fastened
to the tree."

Rollo took hold of it, as his father had directed, and then his father
told him to shut his eyes. When his eyes were shut, so that he could not
see, his father said that he was going to strike the string, at his end
of it, with his pencil-case, and he asked Rollo to observe whether he
could feel any motion.

Rollo held very still, while his father struck the string; and
immediately afterwards he called out, "Yes, sir." Then his father struck
the string again, several times, and every time Rollo could feel a
distinct vibratory or quivering motion, which was transmitted very
rapidly through the string, from one end to the other; although, as the
string was fastened by both end